(Nanowerk Spotlight) With increasing sensitivity, electrical, mechanical and optical sensors are able to detect low molecular weight chemical and biological analytes under ever more dilute conditions. At the same time, though, researchers want to keep the sensing process as simple as possible without complex functionalization and complicated preparation steps for the in situ detection.
Researchers have now designed novel graphene-gold metasurface-based biosensing architectures that make possible extreme phase singularities due to a strong field enhancement on the graphene-gold interface. These sensors could find applications in drug screening, water monitoring and cancer diagnostics.
They successfully demonstrated that graphene layers could accommodate surface plasmons and strongly enhance the electric field of the excited surface plasmon waves. Moreover, through the coupling of localized surface plasmon resonance (LSPR) of gold nanoparticles with the surface plasmon waves on the graphene surface, the electric field could be further enhanced.
Reporting their findings in the September 9, 2015 online edition of Advanced Materials ("Graphene–Gold Metasurface Architectures for Ultrasensitive Plasmonic Biosensing"), the international team, led by professors Ken-Tye Yong from CINTRA and Andrei V. Kabashin from Aix Marseille University, demonstrated a label-free optical detection by using these plasmonic metasurfaces for an extreme low concentration of single stranded DNA down to 10-18 M.
"More importantly, the molecular weight of the 24-mer targeted DNA is only 7.3 kDa," Dr. Shuwen Zeng, a research fellow at CINTRA Laboratory at Nanyang Technological University in Singapore, and the paper's first author, points out. "To detect such small molecules is much more difficult than detecting large molecular weight molecules such as bovine serum albumin (BSA) proteins (∼100 kDa)."
Zeng also notes that by using the team's optimized plasmonic metasurface, they were able to achieve attomolar detection for DNA even without any complex labels, just with the monolayer graphene-coated gold thin film.
In their work, the researchers designed two types of graphene-gold metasurface architectures. One is a basic architecture with a layer of graphene deposited on a gold surface. To excite surface plasmon polaritons over the graphene-gold interface, a light beam is passed through a glass prism and reflected from a 50-nm gold film deposited on one of its facets.
The other one is an advanced nanoparticle-enhanced architecture that uses gold nanoparticles as SPR amplification tags as shown in Figure 1:
Figure 1. Designs of graphene–gold metasurface architectures. a) Basic architecture with a layer of graphene deposited on the gold surface. To excite surface plasmon polaritons over the graphene–gold interface, a light beam is typically passed through a glass prism and reflected from a 50 nm gold film deposited on one of its facets. b) Advanced nanoparticle-enhanced architecture with the employment of gold nanoparticles as SPR amplification tags. c) Finite element analysis (FEA) simulations of resonant monolayer graphene-coated gold sensing film: Electric field in y-component, showing angle of incident light ∼52° and clear evanescent field at the sensing interface. d) FEA simulations of resonant spherical gold nanoparticle coupling to the monolayer graphene-coated gold sensing film: Norm of the electric field with gold nanoparticles (diameter is 30 nm, distance from the sensing film is 5 nm). e) Cross-section plots for total electric fields along y = 0 with different number of graphene layers L. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"Both of our theoretical and experimental results show that even graphene layers can accommodate surface plasmons and strongly enhance the electric field of the excited surface plasmon waves," says Zeng. "Moreover, through the coupling of localized surface plasmon resonance (LSPR) of gold nanoparticles with the surface plasmon waves on the graphene surface, the electric field could be further enhanced."
The strong electric field at the sensing interface leads to the ultrahigh sensitivity of the setup down to 1×10-18 M for 7.3 kDa 24-mer single-stranded DNA at a signal-to-noise ratio of 3:1.
The scientists note that most of the current research on graphene plasmonic sensors is based on theoretical studies only and without systematic optimization of the sensing model. But in their new work, the team not only systematically optimizes the sensing model with two different methods – Transfer-matrix method (TMM) and Finite element analysis (FEA) – they also experimentally demonstrate their optimized structures and obtain the benchmark results.
"Another feature to our work is the phase singularity we employed for detecting the plasmonic signal changes which is quite different from the conventional angular/wavelength-resolved plasmonic sensors," says Zeng. "And the phase singularity induced by the charge transfer from graphene to the gold thin film resulted in a complete light energy transfer (∼100%) to plasmon resonance energy, leading to a strong electric field enhancement at the sensing interface and thus the ultrahigh sensitivity of the setup."
Although the researchers only used DNA molecules as a targeted example, in future work the sensing target could be further extended to chemical targets like explosive TNT and other small cancer biomarkers like cysteine-rich intestinal protein (CRIP).
Also, the optical setup and the sensing film could be further miniaturized to a scale like the size of a small portable box for military, theranostic and environmental monitoring purposes.
In the next stage, the team plans to focus on the specificity of the graphene plasmonic metasurfaces.
"Since the target molecules with aromatic rings all could be adsorbed to the graphene sensing surface through pi-stacking force, we may consider to use a sandwich structure to improve the specificity of the setup," explains Zeng. "For example, the graphene metasurface could be first adsorbed with capture ssDNA I and after saturation, the specificity could be tested by the respective flow of the complementary target ssDNA and the uncomplimentary ssDNA to the sensing surface."
"Also, gold nanoparticles are known to be easily functionalized with thiol-capped capture ssDNA II due to their high affinity to the thiol groups. Thus, the sandwich structures with target molecules between the gold nanoparticles and the graphene-coated film could also assure the high specificity."
The research topic of plasmonic metasurfaces is still in the infant stage.
"One challenge" notes Zeng, "is that the absorption rate of monolayer graphene is 2.3% which is a little bit low for the optical absorption, we may search for new materials from the transition metal dichalcogenide (TMDC) like MoS2 monolayers to form the van der Waals heterostructures to compensate this limit. From the literatures, monolayer MoS2 has not only a higher absorption rate (~5%) than graphene, but also has a lower electron energy loss."